Dark data is the inactive or unused information stored in your systems—often...

https://www.list-bookmarks.win/dark-data-is-the-information-your-company-collects-but-doesn-t-use-often

Dark data is the inactive or unused information stored in your systems—often 55% of all data. It increases storage and backup costs, sometimes wasting millions annually, and can expose you to ransomware or breach risks